A mathematical function which reverses another function.
For example the square root function x(1/2) is the inverse function of the square function x2.
Another example is the Natural logarithm (log to the base 'e') which is the inverse function of the exponential function ex.
The function 1/x, also written x-1.
(As it happens, the inverse function of 1/x - in the first sense above - is itself, namely 1/x.)